1. Field of the Invention
The invention relates to charged-particle optical systems such as electron and ion probe-forming systems, electron and ion microscopes, and electron and ion microanalyzers. It relates particularly to high resolution scanning electron microscopes, high resolution scanning transmission electron microscopes, and to high resolution scanning ion microscopes and microanalyzers.
2. Description of Prior Art
Round electromagnetic and electrostatic lenses typically used in electron and ion microscopes and microanalyzers suffer from unavoidable aberrations such as spherical aberration and chromatic aberration. In the case of scanning electron microscopes and scanning transmission electron microscopes which focus the electron beam into a narrow probe, these aberrations cause the smallest attainable probe size to be about 100 times or more greater than the de Broglie wavelength of the electrons, whereas in a highly perfected optical system the size of the smallest attainable probe should be similar to the de Broglie wavelength. In the case of ion microscopes and microanalyzers, the aberrations cause the ratio of the smallest attainable probe size to the de Broglie wavelength of the ions to be typically one million or more. The result is that the ability of present-day electron and ion microscopes and microanalyzers to probe matter directly on the atomic scale is greatly limited.
A promising approach for overcoming the aberrations is to use non-round lenses such as quadrupoles, sextupoles, octupoles, decapoles, etc. These lenses readily produce aberrations of up to third and higher orders with adjustable polarity and magnitude, as described for instance in chapter 41 in a book entitled Principles of Electron Optics by P. W. Hawkes and E. Kasper, hereby incorporated by way of reference. Despite a prevailing belief, held until recently, that aberration correctors are too complicated and cannot be made to work in practice, as for instance expressed in the book by Hawkes and Kasper, fully working aberration correctors have recently been developed. These correctors successfully minimize aberrations such as spherical aberration, for instance as described in an article entitled xe2x80x9cTowards sub-xc3x85 electron beamsxe2x80x9d by O. L. Krivanek, N. Dellby and A. R. Lupini in the journal Ultramicroscopy (volume 78, page 1), hereby incorporated by way of reference. The operation of such aberration correctors is, however, complicated. Although they remove the strong aberrations that are present in round-lens probe-forming systems, they invariably produce a large number (greater than 10) of additional aberrations of different kinds, called parasitic aberrations. The parasitic aberrations arise principally because the elements of a practical corrector cannot be manufactured or aligned to nm-level tolerances, and because magnetic materials used in the construction of the corrector are not homogenous.
The principal effect of each parasitic aberration is to make the probe larger. Suitable auxiliary optical elements can in principle be constructed and energized appropriately to mimmize the effect of an aberration if its type and magnitude are known. Unfortunately, it is typically not possible to determine the type and magnitude of an aberration causing the probe to become larger by examining a single scanned image. Furthermore, in the absence of a means to quantitatively characterize all the parasitic aberrations having an adverse effect on the probe size, the minimization of the deleterious effects of the aberrations cannot be performed, and the overall performance of the optical system typically becomes worse than if no aberration corrector were incorporated.
Since the number of aberrations that need to be considered is large (typically greater than 10), it is highly desirable that the measurement of the aberrations and their minimization be performed automatically. An autoadjusting electron microscope apparatus and method are known from U.S. Pat. No. 5,300,776. This microscope uses a non-scanning parallel illuminating beam and a plurality of beam directions to characterize and automatically adjust the imaging properties of an electron microscope, but the method of the invention is unsuitable for a probe-forming apparatus using a small scanned probe.
Most users of probe-forming electron or ion microscopes and microanalyzers would find significant advantage in a method and an apparatus providing an autoadjusting charged-particle probe-forming apparatus, which minimized all important primary and parasitic aberrations, and was able to adjust its optical elements automatically. They would find significant advantage in the smaller diameter charged-particle probes and improved resolution charged-particle images that such an apparatus would provide, thereby enabling them to explore the structure and composition of matter on the atomic scale with hitherto unattainable resolution and sensitivity. They would also find advantage in the fact that such an autoadjusting charged-particle probe-forming apparatus would allow them to use charged-particle probes of larger angular range without increasing the size of the probe, thereby delivering a larger flux of charged particles to a given area for the purposes of imaging, chemical micro- and nano-analysis, or micro- and nano-machining. Moreover, the users would find significant advantage in an autoadjusting charged-particle probe-forming apparatus able to perform the required set-up in a few seconds to a few tens of seconds using high-end personal computers and a wide variety of practical samples.
Accordingly, several of the objects and advantages of the present invention are: to provide a method and an apparatus for an autoadjusting charged-particle probe-forming apparatus that can characterize the aberrations of a charged-particle optical system quantitatively and minimize the effects of the aberrations automatically, to provide charged-particle probes of smaller diameter and of larger current densities than hitherto possible, and to provide charged-particle images of improved resolution.